This application is a continuation of the National Stage of International Application No. PCT/US00/02649 filed Feb. 2, 2000, which claims the benefit under 35 USC 119(a)-(d) of Chinese Patent Application No. 99102207.6 filed Feb. 12, 1999.
The present invention relates to magnetic material compositions and a process for their manufacture, and more particularly performance magnetic material of the iron-boron-rare earth type (Fexe2x80x94Bxe2x80x94R).
Use of high performance permanent magnets of Fexe2x80x94Bxe2x80x94R type, where R is a rare earth element containing high concentrations of the element Neodymium (Nd) has become common in industry since the early 1980""s. For example, computer hardware manufacturers who manufacture small footprint, large capacity computer data storage and retrieval hardware need permanent magnets in such devices. Due to severe size and weight restrictions inherent in such data storage devices, the permanent magnets contained therein must be relatively small yet have very strong magnetic properties to generate the required magnetic field. This has necessitated the use of very high performance Fexe2x80x94Bxe2x80x94R rare-earth permanent magnets within such devices.
In addition, medical diagnostic devices in the medical field, namely, magnetic resonance imaging (MRI) devices employ vast quantities (up to 1.5 tons) of permanent magnetic material, typically Fexe2x80x94Bxe2x80x94R magnets that contain high percentages of the rare earth Nd as the rare earth component.
Accordingly, due to the sale of these devices since the early 1980""s employing Fexe2x80x94Bxe2x80x94R permanent magnets wherein the rare earth component is principally comprised of Nd, the worldwide demand for Nd has increased. As a result, the cost of the raw material Nd used in manufacture of such permanent magnets has greatly increased.
A real need has arisen to develop Fexe2x80x94Bxe2x80x94R magnets of substantially equal performance, which utilize less Nd to thereby reduce the cost of the manufacture of such magnets and the devices that contain such magnets.
Permanent magnets of the Fexe2x80x94Bxe2x80x94R type, where R is one or more rare earth elements of which at least 50% of R is Nd and/or Praseodymium (Pr), are known. For example, U.S. Pat. Nos. 4,684,406 and 4,597,938 both teach a high performance sintered permanent magnetic material of the Fexe2x80x94Bxe2x80x94R type. Such patents teach a high performance magnet consisting of, by atomic percent, (i) 12.5%-20% R wherein R is at least one rare earth element (selected from the group consisting of Nd, Pr, La, Ce, Tb, Dy, Ho, Er, Eu, Sm, Gd, Pm, Tm, Yb, Lu and Y) and at least 50% of R consists of Nd and/or Pr; (ii) 4-20%B; and (iii) the balance Fe with impurities. Likewise, as may be seen from U.S. Pat. No. 4,597,938, U.S. Pat. No. 4,975,130 and U.S. Pat. No. 4,684,606, such patents teach a process of preparing such a permanent magnet comprising forming powders of alloys of the above composition; melting same to form an ingot; pulverizing the ingot to form an alloy powder having a mean particle size from 0.3 to 80 microns; compacting such powder at a pressure of 0.5 to 8 Ton/cm2; subjecting the compacted body to a magnetic field of about 7 to 13 kOe; and lastly sintering at a temperature between 900 to 1,200xc2x0 C. (preferably 1,000 to 1,180xc2x0 C.). A permanent magnet prepared in the above fashion specifically comprised of 77 Fe-9 B-9 Nd-5 Pr (wherein Nd and Pr together comprise the rare earth component), sintered at 1,120xc2x0 C. for four hours in an inert atmosphere can acquire a high maximum energy product (BH)max of approximately 31.0 MGOe. Likewise, a permanent magnet comprised of 79 Fe-7 B-14 Nd, sintered at 1,120xc2x0 C. for one hour at (atmosphere), can acquire a maximum energy product (BH)max of approximately 33.8 MGOe (ref. Table 1, U.S. Pat. No. 4,975,130). A sintered permanent magnet comprising 77Fe-7B-16Pr, sintered at 1,040xc2x0 C. in a vacuum at 1xc3x9710xe2x88x924 torr for two hours can be produced having a maximum energy product somewhat less, namely, 24.5 MGOe.
None of these prior art patents disclose or suggest what significance, if any, the amount of Ce present or the concentration of Ce as part of R may have on Fexe2x80x94Bxe2x80x94R magnet performance. Nor does the prior art teach or suggest ranges of concentrations of Ce which may form part of the rare earth component of an Fexe2x80x94Bxe2x80x94R magnet in substitution for Nd which will give equal or better magnetic performance of an Fexe2x80x94Bxe2x80x94(Nd and/or Pr) permanent magnet.
Applicants have discovered that relatively small percentages of Cerium, which in any event usually naturally occur in rare earth deposits containing Nd, may be included in certain defined percentages as part of the rare earth component xe2x80x9cRxe2x80x9d of an Fexe2x80x94Bxe2x80x94R magnet. R additionally comprises 70-76% Pr, 29.8-23.8% Nd and up to and including 5% Ce with no significant or only slight decrease in the magnetic performance of the resultant permanent magnet.
The applicants have further discovered that when certain low percentages of Cerium (0.5% wt. of R) are used in manufacture of an Fexe2x80x94Bxe2x80x94R permanent magnet in substitution of Nd, further substitution of Nd may further be made with Pr up to 76% which further substitution, particularly at Pr=75%, will not reduce, and indeed appears to substantially equal or even enhance, the magnetic performance characteristics over a magnetic material made up of the same total percentage of rare earth elements but lacking Ce, or having Ce but having higher concentrations of Nd.
By substitution of portions of Nd with specific small percentages of Ce and greater amounts of Ce in accordance with the invention herein disclosed, significant cost savings can be achieved in the manufacture of high performance permanent magnets of the Fexe2x80x94Bxe2x80x94R type, while substantially maintaining the magnetic performance of the magnet. Even in respect of certain concentrations of added or entrained Ce that may cause a reduction in the magnetic performance of the Fexe2x80x94Bxe2x80x94R magnetic material as compared with Fexe2x80x94Bxe2x80x94R magnetic material that employs substantially pure Nd, additional magnetic material having Ce and Pr as described herein can be used so as to make up the deficit in strength of magnetic field required in an MRI device.
Accordingly, in one broad aspect the applicants invention comprises a high performance permanent magnetic material of the Fexe2x80x94Bxe2x80x94R type, said material essentially consisting of:
(i) 13-19 atomic % R, where R comprises a mixture of rare earth elements Nd, Pr, and Ce wherein Ce is between approximately 0.2% and no more than 50 wt % of R and Pr is between 70-76% and 29.8-23.8% Nd;
(ii) 4-20 atomic % B;
(iii) the balance comprising Fe with impurities.
In preferred embodiments, the Fexe2x80x94Bxe2x80x94R magnet of the present invention essentially consisted of, by atomic %, 15-16% R, with Ce comprising 0.5-3 wt. %, and preferably 0.5%, with the remainder of R essentially consisting of Pr and/or Nd, preferably in the order of about 71.6% Pr and 24.9% Nd, i.e. a 3:1 ratio.
The present invention further comprises a sintered permanent magnetic material of the Fexe2x80x94Bxe2x80x94R type when made in accordance with the following process, namely:
(a) preparing a metallic powder having a mean particle size of 0.3-80 microns, said metallic powder formed from a composition essentially consisting of 15-16 atomic % R, wherein R essentially consists of the light rare earths Nd, Pr, and Ce, wherein Ce is between 0.2-5.0 wt. % of said R, the balance of R essentially consisting of 29.8-23.8% Nd and 70-76% Pr; 4-8 atomic % B, and at least 52 atomic % Fe;
(b) compacting said powder at a pressure of at least 1.5 ton/cm2;
(c) sintering the resulted body at a temperature of 900-1200 C in a non-oxidizing or reducing atmosphere.
In addition, the applicants have found that while adding Cerium generally tends to decrease magnetic performance of Fexe2x80x94Bxe2x80x94R magnets having only Nd, by substituting Pr for Nd where Ce concentration is low will cause substantial restoration of lost magnetic performance. Accordingly, the applicant has found that using low concentrations of Ce (0.5% wt.) of R with the balance of R essentially consisting of approximately 74.6 wt. % Pr and approximately 24.9 wt. % Nd, wherein the aforementioned process is carried out such will produce a permanent magnet having magnetic performance criteria, namely Hci and (BH)max values, substantially equal to or somewhat in excess of an Fexe2x80x94Bxe2x80x94R magnet wherein the R component is comprising of only Nd and/or Pr.
The invention further comprises a method for producing sintered permanent magnets. In particular, the invention also comprises a process for preparing a sintered permanent magnetic material of the Fexe2x80x94Bxe2x80x94R type, said process comprising:
(a) preparing a metallic powder having a mean particle size of 0.3-80 microns, preferably no more than 4.0 microns, wherein the metallic powder essentially consists of a composition consisting of 15-16 atomic % R, wherein R essentially consists of the light rare earths Nd, Pr, and Ce, wherein Ce is between 0.1-5.0 wt. % of said R and preferably 0.5% of R, and preferably approximately 74% Pr and 25% Nd; 4-24 atomic % B and preferably 6.5 atomic % B; and at least 52 atomic % Fe and preferably 78 atomic %;
(b) compacting said powder at a pressure of at least 1.5 ton/cm2;
(c) sintering the resulted body at a temperature of 900-1200 C in a non-oxidizing or reducing atmosphere.